1. Field of the Invention
The present invention relates to halftone printing and, more particularly, to printing with multi-signal transmission ink.
2. Brief Description of Prior Developments
In some applications indicium, such as one dimensional or two-dimensional bar codes, are printed on the items, such as mail pieces. For example, bar codes are often used on a mail piece such as with, or in, postage meter indicium. A black color is preferably used for printing two-dimensional bar codes and is the preferred color for optimal contrast and readability. One problem with the printing of black two-dimensional bar codes is that they can lead to easy duplication by available desktop printers or copiers. The black prints can easily be copied by any black-and-white copier or scanned and reprinted easily by available printers. To overcome these problems, in some applications it is desirable to pass some information covertly or at least provide a system for verifying the authenticity of the printed indicium. For example, it may be desired to verify the authenticity of postage on a mail piece. Color fluorescent inks are known such as described in U.S. patent application publication Nos. US 2002/0195586 A1, US 2003/0005303 A1, and US 2003/0041774 A1, which are hereby incorporated by reference in their entireties. Invisible ink jet inks are also described in U.S. patent application Ser. No. 10/331829 filed Dec. 30, 2002 which is also hereby incorporated by reference in its entirety.
A problem exists with regard to use of color fluorescent inks in that the inks are more expensive than conventional non-fluorescent inks. In addition, it is customary to increase ink usage for achieving better contrast and surface coverage. This creates the problem of increased costs. There is a desire to reduced costs associated with printing of more expensive inks, such as fluorescent or phosphorescent inks. However, there is also a desire to ensure sufficient quality of a printed indicium such that the indicium, or at least a portion of the indicium, can be machine readable.